Change Screen Message

I have created a menu item titled "Event Registration", that will open the Event Category list, but only for registered users. If someone who has not yet registered clicks on the menu item "Event Registration" this message appears:

Message
The category you are trying to access is invalid or you are not authorized to access that category

Is this a Joomla message or an Event Booking message ? I would like to change the message to something descriptive such as "You are not authorized to view this area, please log in first". I have not been able to locate where this wording is stored so that I can make that change. I have no problem changing a php file, etc. on the backend if necessary, just trying to determine which file I would need to alter.

This would be a Joomla message.
You can change it by:
1) going to [Extensions -> Language(s) -> Overrides].
2) clicking [New] to add a new override.
3a) I entered the string you quoted into the Search box (with "Search For" set to "value"), and came up with a hit on the language constant "EB_INVALID_CATEGORY_OR_NOT_AUTHORIZED". So...
3b) enter "EB_INVALID_CATEGORY_OR_NOT_AUTHORIZED" into the [Language Constant] field, and modify the [Text] to whatever you'd like.

Great, thank for the reply.